Mineraloid/Anthropogenic (Technofossil)

Ferruginous Slag (Iron Smelting Byproduct)

Anthropogenic Vitreous Silicate (often containing Fayalite: Fe2SiO4)

Hardness: 5-6 (Mohs); Color: Dark brown, black, or rusty grey; Luster: Glassy to dull; Structure: Amorphous/Vesicular (contains air bubbles/pitting); Specific Gravity: 2.8 - 4.0 (denser than most natural rocks).

Hardness: 5-6 (Mohs)
Color: Dark brown, black, or rusty grey
Luster: Glassy to dull

Identified March 29, 2026More mineraloid/anthropogenic (technofossil) →

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Physical properties

Formation & geological history

Formed as a byproduct of historical or industrial iron smelting. Impurities (silica) are separated from iron ore using heat and flux, cooling rapidly into a glass-like material. These are often found near old railroad beds, iron works, or colonial furnace sites.

Uses & applications

Historically used as road ballast, railroad fill, or insulation. Today, it is primarily of historical interest or used in some industrial abrasives and roofing granules.

Geological facts

Slag is frequently mistaken for meteorites due to its dark surface and pitted (vesicular) texture. However, the presence of spherical bubbles (vesicles) is a definitive sign of terrestrial gas escape during cooling, which meteorites do not have.

Field identification & locations

Identify in the field by looking for 'pin-hole' bubbles and glassy edges when broken. It is often found in areas with past industrial activity or used as fill dirt. It may be slightly magnetic if iron remains trapped inside.

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